Generally, an ink-jet printing apparatus has a carriage holding a printhead and an ink tank, conveyance means for conveying a print medium such as a print sheet, and control means for controlling these elements. In the ink-jet printing apparatus, printing is performed by scanning an ink-jet printhead (hereinafter referred to as a “printhead”), having plural ink discharge nozzles (hereinafter referred to as “nozzles”) to discharge ink droplets, in a direction (main scanning direction) perpendicular to a print medium conveyance direction (subscanning direction) while ink is discharged to the print medium. At this time, as a large number of nozzles to discharge ink are arrayed on a straight line in the subscanning direction, printing is performed for a width corresponding to the number of nozzles by scanning of the printhead on the print medium once. Accordingly, the printing speed can be easily increased by increasing the number of nozzles and increasing the printing width of the printhead.
Further, in a printhead where plural printing elements are arrayed in a line, these plural printing elements are divided into plural groups, and the plural groups are time-divisionally driven, sequentially, thereby a printing operation is performed. The number of concurrently-driven printing elements and the number of nozzles to perform concurrent ink discharge are determined in accordance with the number of groups of printing elements.
In the ink-jet printing apparatus, it is assumed under the condition that an environmental temperature and the temperature of the printhead are constant that the same printing density can be obtained by applying the same amount of energy to the printing elements. However, in a case where plural printing elements are divided into plural groups and the groups are time-divisionally driven as described above, the number of concurrently-driven printing elements within one group changes in accordance with an image signal inputted into the printhead. If the number of concurrently-driven printing elements is increased, the amount of an electric current which flows through a common conductor to supply drive power to the plural printing elements is increased.
As a result, a drive voltage applied to the respective printing elements drops, and the energy applied to the printhead is changed, thus a variation occurs in the printing density.
Assuming that a resistance value of the common conductor is R, and a current which flows through one printing element is I, the voltage drop (Vdrop) is expressed asVdrop=R*IAccordingly, in a case where n printing elements are concurrently driven, the voltage drop (Vdrop_n) is expressed asVdrop_n=R*nI.
When all the printing elements belonging to a time-divisionally driven group are driven, i.e., when the number of concurrently-driven printing elements is a maximum (when the drive voltage drop is maximum), in order to realize stable ink discharge, the energy applied to the printhead is determined in consideration of the maximum number of concurrently-driven printing elements. However, in the case where the energy applied to the printhead is determined in this manner, if only one printing element is driven, excessive energy is applied to the printing element, which harmfully affects the durability of the printing element.
Conventionally, as a countermeasure to this drawback, Japanese Published Unexamined Patent Application No. Hei 9-11504 discloses measuring the number of concurrently-driven printing elements in one group and determining a parameter for a drive pulse to be applied to the printing elements in correspondence with the measured number. In this manner, the energy applied to the printhead is changed in correspondence with the number of concurrently-driven printing elements, thereby maintaining the durability of the printing elements and stabilizing the printing density.
In recent years, there is an increasing need for higher image quality, and in response to the need, the size of discharged ink droplets is reduced using various methods in the ink-jet apparatus. For example, if printing is performed on a print sheet using small size ink droplets, a high-quality image without graininess can be obtained in a low printing duty area, while in a high printing duty area, as an image with sufficient density cannot be formed by one ink discharge, the image must be formed by more than one ink discharge. As a result, the printing speed is lowered.
Accordingly, to achieve high-speed printing and high-quality printing, a printing apparatus which forms an image using ink droplets in plural sizes is proposed.
Assuming that a heater resistor of a printing element to discharge a large size of ink droplet is r1, that of a printing element to discharge a small size of ink droplet, r2, a drive voltage, VH, and a resistance value of a common conductor to supply drive power, R, an electric current I1 which flows through the heater resistor r1 and an electric current I2 which flows through the heater resistor r2 are expressed asI1=VH/r1, and I2=VH/r2,and the respective voltage drops (Vdrop1 and Vdrop2) are expressed asVdrop1=I1*R, and Vdrop2=I2*RThat is, the voltage drop values are different.
FIG. 12 is a graph showing the relation between the number of concurrently-driven printing elements and the voltage drop.
In FIG. 12, 1200a denotes a voltage drop by the heater resistor r1, and 1200b, a voltage drop by the heater resistor r2. The difference between these voltage drops increases in correspondence with the number of concurrently-driven printing elements.
In this manner, in a printing apparatus which forms an image using ink droplets in plural sizes, the voltage drop values, which occur in the printing elements used for discharging the respective size ink droplets, are different. Accordingly, it is desirable to set an optimum drive parameter to define the drive pulse to be applied to the printing elements in correspondence with the number of concurrently-driven printing elements.